Field of the Invention
The present invention pertains to the art of transmitting and receiving electromagnetic power using antennas to receive electromagnetic energy and, more particularly, to rectifying antennas that convert electromagnetic energy into direct electrical current.
Discussion of the Prior Art
Generally, a rectifying antenna, otherwise known as a “rectenna”, is an antenna designed to convert electromagnetic energy, preferably microwave energy, into direct (DC) electrical current an thus acts as an energy converter. An example of an early use of a rectenna can be found in a crystal radio which converts RF energy, i.e., radio waves, into an electric current which is sent to a speaker to produce sound. Perhaps one of the most simple of rectennas is formed by placing a Schottky diode between the dipoles of an antenna. Often, rectennas are formed by multiple Schottky diodes linked together. Under certain favorable conditions, such antennas have been known to convert microwave energy into electrical current with an efficiency of over 90%.
Because rectennas are able to transmit power over a distance with high efficiency, they are commonly employed in solar powered satellites. Additionally, proposals have been made to use rectennas as part of a system to transfer energy to flying machines. For example, U.S. Pat. No. 3,434,678 discloses a combined antenna and conversion mechanism for the reception of beamed high frequency energy. Specifically, a full-wave bridge connected diode network is used as a rectenna to provide power to a helicopter. However, such an arrangement requires that the received energy be polarized and that the orientation of the applied field be maintained parallel to the orientation of the rectenna dipoles in order to maintain high efficiency in collecting power.
Another broadband rectenna system is formed from an array of traces loaded by Schottky diodes and is often referred to as a “Dense Diode Grid Array.” With initial reference to FIG. 1, an energy converter such as a rectifying antenna or rectenna for receiving electromagnetic energy constructed is generally indicated at 10. Rectenna 10 has a positive terminal 12 and a negative terminal 14 which collectively constitute an electrical output. Rectenna 10 also has a grid array 20 that is located along a surface 30 of a substrate 35. Grid array 20 includes a grid pattern 40 of diodes 41. Diodes 41 are divided into a first group, represented by diode 44, of electrodes that extend in a first direction and a second group, represented by diode 46, of electrodes that extend in a second direction, perpendicular to the first direction. For the sake of convenience, the first and second directions are referred to as horizontal and vertical respectively, although it should be noted that Rectenna 10 might be positioned in various different orientations.
Referring now to FIG. 2, there is shown a portion of grid array 20. Two diodes 50, 51 are linearly aligned and arranged in series along a conductive horizontal trace 55 on surface 30. Similarly, two diodes 60, 61 are arranged in a linear fashion along a conductive vertical trace 65, also on surface 30. Vertical and horizontal traces 55, 65 share a common node 70.
As shown in FIG. 3, an electromagnetic field vector 100 is shown rotating by arrow 101 about an axis of rotation 104 (extending into and out of the page) and in a plane 105 extending in a vertical direction along vertical axis 106 and a horizontal direction along horizontal axis 107. Rotating electromagnetic field vector 100 is used to represent a randomly polarized electromagnetic wave of incident RF energy impinging on array 20. Rotating electromagnetic field vector 100 is continuously split into two orthogonal components 100V, 100H as vector 100 completes a cycle of rotation about axis 104. In this case, vertical component 100V of field vector 100 aligns with diodes 44 arranged vertically and, in a similar manner, horizontal component 100H aligns with diodes 46 arranged horizontally. Of course, grid array 20 will harvest energy from incident RF energy even if diodes 44 and 46 are not arranged at right angles, but the efficiency of harvesting will drop.
Turning back to FIG. 1, grid array 20 includes an entire dense rectangular grid pattern of diodes 44, 46 connected by nodes such as node 70. As such, numerous vertical traces 120 and horizontal traces 130 connect diodes 44, 46 to positive and negative terminals 12, 14. When vertical and horizontal traces 120, 130 are so arranged in array 20, the entire array 20 reacts to both components 100V, 100H of electromagnetic field vector 100. Electromagnetic forces are combined in array 20 giving a large DC voltage between terminals 12, 14, with junction capacitance minimizing any ripple voltage. Vertical and horizontal, diode-loaded, conductive traces 120, 130 alternately forward bias during each half cycle of rotating vector 100, allowing free charges to store up and aid in biasing adjacent orthogonal, diode-loaded traces 55, 65 during the next half of the cycle. Therefore, the dense rectangular grid pattern 40 of orthogonally oriented diodes 44, 46 will act as a half-wave rectifying element when illuminated by the incident RF energy represented by vector 100.
In general, such prior art broadband rectennas exhibit low efficiencies when harvesting RF energy from a randomly polarized illumination source. With the above in mind, there is considered to be various advantages associated with further developments for rectennas. In particular, there is seen to be a need in the art for a rectenna array that has increased power and efficiency along with a wide frequency response that is efficient at harvesting energy from a randomly polarized illumination source.